# Homework Help: Heat conductivity

1. Sep 7, 2004

### TSN79

A tank holds water at 60 degrees C. The outside temp. is 10. The thickness of the walls is 12mm. My goal is to find the temp on the wall on the outside of the tank. I've got some values I don't know the english name for, but hopefully you'll understand by the units.
$$\lambda$$ for steel is $$50\frac{W}{mK}$$
h between water and tank is $$2700\frac{W}{m^2K}$$
h between air and tank is $$15\frac{W}{m^2K}$$

How do I go about it?? I also need to find the effect loss...

2. Sep 7, 2004

### Gokul43201

Staff Emeritus
How to go about it : The heat flowing per unit area from the water, Q, is equal to the heat received per unit area by the outside air (and is also equal to the heat conducted per unit area through the tank wall). Calculate each of these heat flows and equate them to each other.

I've already said too much... in the future, if you want help, you must show what you've tried and where you are stuck.

3. Sep 8, 2004

### TSN79

First I found the value which we in norwegian denote U [W/(m^2*K)]
$$U=\frac{1}{(1/h_1)+(\delta_1/\lambda_1)+(1/h_2)}$$
Putting in the values this gives me U=14,86 W/(m^2*K)
Then I want to use the equation
$$\dot{Q}=U \cdot A(T_1-T_2)$$
where A is the area of the tank wall, T1 and T2 are the temp. of the water and the air and Q is the energy that passes through the wall. The problem is that I don't have the area, I only have the thickness of the wall, 0,012m. If I did know A, then I could calculate Q, and use this equation
$$\dot{Q}=h_1 \cdot A(T_1-T_w_1)$$
to find $$T_w_1$$ which is the temp. at the outside of the tank wall.
The funny thing is that if I follow this procedure and use my value for thickness istead of area, the answer comes out right (!), though the unit of the answer isn't correct. By the way, the answer is 59,5 degrees C.
What am doing wrong? Is there some other equation I should be using?